NASA Technical Memorandum 88966

Tribological Properties of Polymer

Films and Solid Bodies in a

Vacuum Environment

Robert L. Fusaro

Lewis Research Center

Cleveland, Ohio

Prepared for the

1987 Annual Meeting of the American Society of Lubrication Engineers

Anaheim, California, May 11-14, 1987
TRIBOLOGICAL PROPERTIES OF POLYMER FILMS AND SOLID BODIES

IN A VACUUM ENVIRONMENT.

Robert L. Fusarc

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

Abstract

The tribological properties of ten different polymer based materials were

evaluated in a vacuum environment to determine their suitability for possible

lubrication applications in a space environment, such as might be encountered

on the proposed space station. A pin-on-disk tribometer was used and the

polymer materials were evaluated either as solid body disks or as films

applied to 440C HT stainless steel disks. A 440C HT stainless steel

hemispherically tipped pin was slid against the polymer materials. For

comparison, similar tests were conducted ‘in a controlled alr atmosphere of

z 50 percent relative humidity air. In most instances, the polymer materials

e

m

& lubricated much better under vacuum conditions than in air. Thus, several of

the materials show promise as lubricants for vacuum applications. Frlctlon

coefficients of 0.05 or less and polymer material wear rates of up to 2 orders

of magnitude less than found In air were obtained. One material showed

considerable promise as a traction drive material. Relatively high friction

coefficients (0.36 to 0.52) and reasonably low wear rates were obtained ‘in

vacuum.

INTRODUCTION

The construction and use by the United States of a space station ~n the

1990’s could mean that the trlbologlcal problems encountered in the past on

space vehicles wI1l become even more complex and dlfflcult. Most lubricating

s~tuations to date on space vehicles have been solved either by us~ng low

vapor pressure oils and greases or by using solid lubricants. Oils and
greases have their drawbacks because even low vapor pressure oils can

evaporate and contaminate nearby sensitive surfaces -such as optical devices.

Solld lubricants have their limitations because they have generally higher

friction coefficients than oils and because of finite endurance l’ives. Also

some solid lubricants, which are used in terrestrial applications, do not

function well in a vacuum, e.g., graphite.

The challenge wilJ be to find new solid lubricant materials which w’111 be

able to operate for long slidtng durat~ons ~n the vacuum of space and under

the changing temperature conditions of day and night. Three principal classes

of solid lubricant materials have been used to date: polymers, such as PTFE;

dlchalcogenides, such as molybdenum disulfide (MoS2); and soft metals, such

as lead (~).

It is the author’s belief that polymer and polymer composite materials,

especially the class of polymers called polymides are the best prospects to

meet the new lubrication c~llenges. In previous studies (2-4), -— it was

observed that the frlctlon and wear of some polylmfde films dropped

dramatically as the temperature was raised above a certain level in an air

atmosphere. It was postulated that th~s might be due to the resorption of

absorbed water vapor, since polyamides have a free hydrogen bond which tends

to bond water vapor. Getting rid of the water could give the polylmide

molecules the necessary freedom to flow as a thin films on the polymer

surface. Thus, one of the objectives of this study was to determine !f the

frtction and wear of polyimide materials would drop in the absence of air and

water vapor. However, the main objective of the study was to trlbologlcally

evaluate In a vacuum environment several commercially available polymeric

materials (either in the form of a solid body or a film applled to a metal) In

order to determine if any of these materials m~ght have potent~al for future

space station applications. A pln-on-disk apparatus was used and vacuums of

2
either 0.13 Pa (10-3 torr) or 0.00013 Pa (10-6 torr) were employed. For

comparison; slmtlar experiments were also conducted .in a controlled 50 percent

RH air atmosphere.

MATERIALS

Ten different types of polymer materials were evaluated. Seven as solid

body disks and three as films applied to AISI 440C HT (high temperature)

stainless steel dtsks with a Rockwell hardness of C-58. t40st of the materials

are commercially available and can be purchased from a vendor. The solid body

matertals were: a polyphenylene sulflde (PPS) disk reinforced with 40 percent

graphite fibers, a Poly (amide-imlde) d~sk wtth PTFE and graphite powder

additives, a commercially available polylmlde with no addlttves, a PMR-15

polyimide (~) with a 70 percent PTFE powder addlttve, a polyimide made from

the diamine 2,2-his (4-(aminophenoxy)phenyl) - hexafluoropane (4-BDAF) and the

dianhydrtde pyromellitic acid (PMDA) (~,7) (for short this polyimlde w’Ill be

referred to as 100 PMDA), a co-polyimide made from the diamine 4-BDAF and

80 mol % of the dianhydride PMDA and 20 mol % of the dianhydride

benzophenonetetra-carboxilic actd (BTDA) (~,7) (for short this polyimide will

be referred to as 80 PMDA/20 BTDA), and an 80 PMDA/20 BTDA polyimlde

reinforced with 50 percent high modulus graphite fibers (~). The films

evaluated were a 100 PMDA poly’imlde film, a PI-4701 polytmide film (2,3), and

a PI-4701 polyimide film with 50 wt % of thesolid lubricant graphite

fluoride (~).

APPARATUS

A pin-on-disk tr~!~ometer was used to evaluate the polymer materials. The

apparatus used for the alr tests ts descrfbed ~n detail in Ref. 10. A

schemat:c of the test >Declmens is shown ~n Fig. 1. The vacuum tribometer was

essentially the same as the alr tribometer except

were enclosed in a stainless steel vacuum chamber

that the friction specimens

and a magnetic drive

3
coupling was used to rotate the d~sk. The fulcrum of the lever arm used to

apply the load to the pin could be translated in the direction of the lever’s

length, so that the diameter of the circumference of the wear track on the

disk could be changed without removing the specimens from the vacuum. A

sorption pump was used to achieve a pressure of 0.13 Pa (10-3 torr) and an

ionization pump was used to ach!eve a pressure of 0.00013 Pa (10-6 torr) .

PROCEDURE

Specimen Cleanlnq

The AISI 440C HT stainless steel pins and disks were cleaned by washing

with ethyl alcohol and then by scrubbing with a water paste of levigated

alumtna. They were then scrubbed wtth a brush under running tap water to

remove the levigated alumina, rinsed with distilled water, and dried with

clean compressed a’tr.

The polymer solid bodies were scrubbed

rinsed with distilled water,

films were not cleaned since

coat~ng and were not touched

Experimental Testing

and then dr~ed

they were put “

by human hands

w’ith a nonabras~ve detergent,

with clean compressed air. The

n a desiccator immediately after

The specimens were inserted into the tribometer and the chamber sealed.

For the atr tests, a controlled 50 percent relative humidity alr was purged

through the chamber for 15 min before commencing the tests and continuously

throughout the tests. For the vacuum tests, the chamber was held at the

desired vacuum pressure for 15 min before commencing the tests. The disk was

then set Into motion at 100 rpm and a 9.8 N load was gradually applied to the

disk through the pin. The pin slld against the disk at a constant diameter

which could be varied from 1.5 to 6.0 cm.

After var~ous intervals of slidlng, the tests were stopped and the

specimens removed from the trlbometer. The pln and disk rubbing surfaces were

4
then studied by optical microscopy and surface profiles of the disk weart rack

were taken: From these profiles, polymer wear track cross-sectional area was

measured and wear rate calculated. The specimens were reinserted into the

tribometer and the above

constant wear rates were

procedure repeated. The tests were terminated after

ascertained or failure occurred (films worn through

to the metal substrate and high friction obtained).

RESULTS ANO DISCUSSION

Friction Coefficient

Previous Work - In previous studies (~-~), it was observed that for some

polyimide materials, the friction coefficient tended to decrease as a function

of increasing temperature. It was postulated that the reason for this was

that water vapor hydrogen bonded to the polyimide chains at lower temperatures

and constrained the chains freedom “to unfold” into an extended molecular

chain type of structure (~); and thus brittle fracture and higher friction

occurred under these sliding conditions. By increasing the temperature to a

h’igher level, the H20 molecules either debonded or their influence became

negligible, and friction and wear dropped up to an order of magnitude. When

this happened very thin plastically flowing surface layers were observed on

the film wear tracks and on the pin slidlng surfaces.

Experiments in dry argon (20 ppm, H20), dry air (100 ppm, H20), and

>

moist air (10 000 ppm H20) (~) indicated that this transition from high

friction-high wear to low friction-low wear occurred at a lower temperature as

the amount of water vapor in the air decreased, and indicating that even a

very small amount of water vapor could affect the friction and wear of

polyamides. Thus, in addjtton to evaluating several polymer materials for

possible space usage, an objective of this study was to determine if low

frictjon and low wear could be obtained at ambient temperature jn the complete

5
absence of water vapor. The following ‘IS a discussion of the frictional

characteristics of the polymer materials evaluated in air and in vacuum.

Polyphenline Sulflde Olsk Reinforced with Graphite Fibers - F’tgure 2

plots friction coefficient as a function of slidfng durat~on for the

polypheneline sulfide solid disk which was reinforced with 50 percent graphite

fibers and evaluated in air and In a 0.13 Pa vacuum. For both environments,

the frfction coefficient started out at 0.30 and then rose w~th sliding

duration to a value of about 0.44. In vacuum, after about 40 kc of sliding,

there was a sharp drop in friction, and the value eventually leveled off and

remained stable at about 0.03. There was also a drop in air after about

200 kc of slidtng, but the value dropped off only to 0.30. Only the first

400 kc of sliding are shown in the f~gure but the air test was for 1300 kc and

the vacuum test for 925 kc and In both instances the friction remained stable.

Poly(amide-lmlde) with PTFE and Graphite Disk - Figure 3 compares the

frict’ton properties of a Poly(amide-lm!de) solid polymer disk with PTFE and

graphite add~t~ves. The frlct+on coefficient for this composite started out

at about 0.18 in afr and In vacuum and rose to an average value of about

0.42. In alr the friction remained relatively constant at this value for

1560 kc of slldlng. In vacuum, the friction started to gradually drop after

about 40 kc of sliding and eventually reached a steady-state value of 0.07

after about 83 kc. The friction remained constant at this value until the

test was terminated at 380 kc of sllding.

After the vacuum test on this composite was terminated, instead of

removing the specimens from the tribometer, the pln was reposit~oned to sllde

on a new track on the disk. The frict~on coefficient for this test again

inltlally started out at a value of 0.18, but only rose to a value of 0.29 in

about 18 kc of sl!ding, then It dropped to a value of about 0.07 after 20 kc

6
of sliding. This indicates that a long exposure to the vacuum helped desorb

the water vapor. from the polymer and shortened the run-in per’iod.

Commercially Avatlable Poly’imide Solld Disk - Figure 4 plots friction

coefflc~ent as a funct~on of sliding distance <n atr and ~n vacuum for a

commercially available polyimide solid body. As with the previous two polymer

~terfals, the friction c~effic~ent for the tests in air and in vacuum started

out at the same value, tn this case 0.18. In air, the friction rose to a

steady-state value of 0-45 in about 45 kc of sliding and stayed constant for

the duration of the test 860 kc. In vacuum, the friction coefftclent tended

to oscillate between 0.33 and 0.18 until It finally reached a steady-state

value of 0.03 after 40 kc of sliding. It remained constant at this value

until the test was terminated at 1450 kc of sliding.

Again, instead of removing the specimens from the vacuum trlbometer, the

pfn was repositioned to slide on a new wear track on the dtsk. Figure 4 shows

that the friction coefficient dropped after approximately 7 kc of slid~ng to a

friction coeff~cient value of 0.03. As with the previous test, this indicates

that long exposures to vacuum can help desorb water vapor from the polyimide

and shorten the run-in period.

PMR-15 Polyimlde Disk. with PTFE Additive - Figure 5 plots fr’ictlon

coefftclent as a function of slidlng duration for the PMR-15 polyimide solid

disk wtth a 70 wt % PTFE additive. The frictjon coefficient in air and

vacuum did not start at the same value for this composite. The friction in

air tended to ?ncrease gradually from 0.12 as a funct~on of sliding distance

until it reached a steady-state value of 0.22 after approximately 650 kc of

sliding. After which It remained constant for the duration of the test,

1690 kc. In vacuum, the friction coefficient started out at 0.08 but then

rose to.a steady-state value of 0.12 after about 280 kc of sliding. It

remafned constant at this value for the duration of the test, 1292 kc.

7
This composite material did not produce a low friction coefficient in

vacuum like the previous three materials. It is bel”leved that this might be

due to the large amount of PTFE in the composite. Desorbing the water may

have had a lesser effect due to the fact that the PTFE may have dom~nated the

frtct~on process.

80 PMOA/20BTDA Polyimide Disk - Figure b plots friction coefficient as a

function of sliding distance for the 80 PMDA/20 13TDA polylmide. For the

80 PMDA/20 BTDA polyimlde in air, the friction coeff~cient started out at 0.18

and then rose constantly for 15 kc of sliding until it reached a constant

value of 0.58 (steady-state). In the vacuum, the frlctlon coefficient also

ftrst rose with sllding distance but then after 8 kc of sllding it started to

decrease and eventually after about bO kc of sliding reached a steady-state

value of 0.04. The tests were slid for 540 kc ‘in air and for lb90 kc in a

vacuum of 0.13 Pa.

One additional test was performed on the 80 PMDA/20 BTDA polylmide only.

In that test the ftlm was evaluated under a better vacuum of 0.00013 Pa.

Under the better vacuum, the friction coefftclent started out at the same

value as in the 0.13 Pa vacuum, but ?nstead of increasing with sliding

distance it dropped very quickly (after about 0.4 kc of sliding) to a

steady-state value of 0.02. The friction coefficient remained constant at

this value for 1920 kc of slldlng.

The results thus substantiate the initial postulation that hydrogen

bonded water vapor is responsible for the high frflct’ionobta~ned w~th the

polytmlde type materials. The water appears to be removed under the action of

sllding In a vacuum and the htgher the vacuum, the faster the removal.

80 PMDA/20 BTDA Polylmide Disk Reinforced with Graphite Fibers - Figure 7

shows friction coeffic~ent as a function of sliding distance for an 80 PMDA/20

BTDA polyimide reinforced with 50 wt % of high modulus chopped graphite

8
ffbers. For the first 225 kc of sliding, the friction coefficient in a~r and

in vacuum was nearly the same (a value of about 0.26), then the friction in

vacuum rose slightly to 0.30. The vacuum test was terminated after 300 kc of

sliding due to wear considerations (cracking and spallation of the graphite

fibers) but the air test was continued on to 5400 kc and the wear surfaces

were still In great shape.

100 PMDA Polyimide Solid Disk - Ftgure 8 gives friction coefficient as a

function of sl’idlng duration for a polylmide which did not show a low

steady-state friction coefficient in vacuum, the 100 PMDA polyim~de solid

body . The friction coeff~cient started out between 0.18 to 0.20 In vacuum and

air and rose very quickly to a steady-state friction coefficient that was

greater than 0.40. As the figure shows there was a considerable amount of

slip-stick type of friction which produced a wide variation of friction as the

pin transverse the disk. These two tests were evaluated for 160 kc cf

sllding.

100 PMDA Polyimjde Film - Figure 9 plots friction coefficient as a

function of sliding duration for the 100 PMDA polyimide that was applted as a

film to a 440C HT stainless steel disk. Similar fr~ction characteristics were

obtatned for the film as for the solid body. That ‘Is, there was no dramattc

drop in friction (as was found for some of the other polymers in vacuum) and

that the values of steady-state friction obtained in alr and vacuum were

nearly the same (about 0.40). Though the steady-state friction coeffic~ent

was slightly lower for the film than for the solid body.

PI-4701 Polyimide Film - The fr’iction coeff’iclent of the PI-4701 film

applied to the 440C HT stainless steel disk started out at about 0.28 In air

and In vacuum and then dropped rapidly (Fig. 10). In air the steady-state

value was 0.15 and in vacuum It was 0.04. In air, the film wore through to

9
the substrate in about 30 kc of sliding, but in vacuum It was still

functioning perfectly after 3020 kc of sltding when the test was term’tnated.

?1-4701 Film with a Graphite Fluoride Addltlve - In air, it was found

that a 50 percent weight addition of graphite fluoride powder could

dramatically increase the endurance life and sometimes decrease the frlctlon

coefficient when compared to PI-4701 films alone (~). In vacuum, neither

improved friction coefficients or endurance l~ves were obtained by add~ng

graphite fluortde powder to PI-4701 films. Figure 11 plots friction

coeffic~ent as a functfon of slldlng d~stance for the PI-4701/graphite

fluoride film. The average friction coefficient in vacuum was 0.07 and the

endurance life was 150 kc. The average frlctlon coefficient In alr was 0.16

and the endurance llfe was 500 kc.

Comparison of Friction Coefflctents - Figure 12 compares the maximum,

minimum, and average steady-state frictfon coefficients obtatned for the

10 polymer materials evaluated. For the vacuum tests, when the frict~on

characteristics of the neet polyamides are compared to the same polyimlde with

solid lubricant additives (graphite fibers or CFX), it is found that the

steady-state fr’ict~on coefficients are greater for the polylmldes with the

solld lubricants additives than without. Thus It Is seen that the friction

coefficient of at least some polyamides In vacuum ‘IS so low that they do not

need solid lubricant additives and that the acfdlt~on of solid lubricant

additives can be detrimental.

WEAR

In all instances, the wear to the metallic pins was too low to measure.

Only polished surfaces and transferred polymer material was observed on the

pins. Wear to the polymer dtsks was measurable, however. Figure 13 plots

disk wear track cross-sectional area as a function of sllding duration in

kilocycles for the 80 PMDA/20 BTDA polyimlde disk. Since the wear track

10
circumference and the sliding distance per cycle are the same, the slopeof

the straight line in this figure represents wear volume per unit sliding

distance.

As found for friction

polymers produced a run-in

Compartng F~g. 6 with Fig.

coeff~c~ent, most of the low frtctfon producing

wear interval

13 for the 80

the same perfod of time for steady-state

occur in air and tn the 0.13 Pa vacuum.

found for the test run in the 0.00013 Pa

before steady-state wear took place.

PMDA/20 BTDA polytmide, it took about .

fr’ictton and steady-state wear to

There essentially was no run-in wear

vacuum, although this might be

disputable stnce only two wear points were determined.

Figure 14 plots disk wear track cross-sectional area as a function of

,.

sllding duration for the 100 PMDA polyimide disk. The figure indicates that

the wear ~ncreases at a constant rate as a funct<on of sliding distance and

that little or no run-in wear took place for this polytmide In either air or

In vacuum, since the lines pass through zero.

the short run-in friction that took place with

Figure 15 plots cross-sectional area as a

for the commercially available polylmlde solid

polytmlde had a very long run-in wear period.

before steady-state wear occurred. It appears

No run-in wear corresponds to

this polyimide (Fig. 8).

function of sliding dfstance

d~sk. In vacuum, this

It took about 440 kc of slldlng

that this Is related to the time

in vacuum; stnce when the pln was moved to a new track without removing it from

vacuum after 1450 kc of slidlng, the wear rate obtained was Identical to the

steady-state value. Comparing Fig. 15’to Fig. 4, the run-in periods for

friction and wear In a vacuum do not correlate. Low fr’iction was obtained

after 40 kc of slldlng but It took over 400 kc for the lowest wear rate to

occur.

The following polymer materials showed no or very little run-in wear In

vacuum: the PI-4701 film, the PI-4701 film with CFX additive, the

11
80 PMDA/20 BTDA polytmide with graphite fiber additives, and the 100 PMDA

polyimide film. In vacuum, the polyphenlene sulftde disk with graphite fibers

had a run-in period of about 70 kc, the Poly(amide-’tmide) disk with PTFE and

graphfte addittve had a run-in period of about 80 kc, and the PMR-15 polyimide

disk w~th 75 percent PTFE had a run in period of about 25 kc.

Figure 16 compares the steady-state wear rate values that were determined ‘

for all the polymer materials in air and in vacuum. For the majority of the

polymers, the wear rate in vacuum was wither equivalent or less than that

obtafned ~n air. The two exceptions were the GFRPI composite and the PI-4701

film with the fluorinated graphite (CFX) additive. Reasons for this will be

discussed In a following sectflon.

Figure 17 plots fr~ction coefficient as a function of steady-state wear

rate for all the polymer materials evaluated in vacuum and for 440C HT

stainless steel evaluated unlubricated in air. The wear rates of all the

polymer materials evaluated can be considered excellent when compared to

unlubricated 440C HT stainless steel, whose

10-8M3/M of sliding. It m~ght be mentioned

stainless steel was not evaluated in vacuum

wear rate Is In the order of

that the 440C HT unlubr~cated

but that the frict~on and wear

should be even worse in a vacuum since protective oxide films would not be

formed.

The best low wear/low frlct’ion material in a vacuum environment was found

to be the 80 PMDA/20 BTDA solid polylmide disk. The steady-state wear rate of

this polyimide was about one-fifth that of the PI-4701 film and about

one-tenth that of the PMR-15 film with the PTFE addltlve. The 80 PMDA/20 BTDA

polyimide which was evaluated in the higher vacuum (0.00013 Pa) gave even ldss

wear than the low vacuum (0.13 Pa) test which indicates that the polymers

which work well in a low vacuum w’Ill work even better in a high vacuum.

12
The 100 PMDA polyimide film and soltd body gave high friction but

reasonably good wear protection. A polymer with these characteristics might

be useful for coatings or as solid body rollers for space station tractfon

drives which would be used to position solar arrays, telescopes, etc.

Wear Surface Analysis

Except for the two polymer solid disks which contained graphite ftbers,

the polymer materials did not wear the sliding 440C HT stainless steel pin

counterfaces. Very thin, flowing transfer films were produced on the ptns.

The wear that was produced by the graphite fiber reinforced disks was not

great and hardly discernible. It was more like fine polishlng and the wear

conformed to the radius of the pin hemisphere.

In vacuum, very smooth

without any solid lubricant

of the 80 PMDA/20 BTDA disk

obtained on the PI-4701 and

surfaces were much smoother

wear tracks were observed on the polyamides

addltlves, Figure 18(a) shows a photomicrograph

wear track. Similar surfaces to this were

the commercially available polylmldes. These

than those obta~ned ~n aft- on the same polylmtdes

and it is believed this ‘IS due to polyimlde’s ability to

a thin film in the absence of water vapor. Figure 18(b)

photomlcrograph of the wear surface of the 100 PMOA poly

wear surface was also found on the 100 PMDA polyimide so”

plastically deform as

shows a

mide film. A slmllar

id disk. In general,

the wear surface was smooth but there are many large ptts on the wear track.

These may have been caused by the h~gh friction between the sllding surfaces.

The powdered solid lubricants which were added to the polymers, tended to

produce wear track surfaces that were not as smooth as the neet polymers.

Figure 19(a) shows a photomicrograph of the PMR-15 polyimide w~th the PTFE

addit~ve. The surface looks smooth but the surface profile of it shows many

voids. The smooth Iooklng nature ‘is due to the transparency of the 70 percent

PTFE additive in the polyimlde.

13
Figure 19(b) shows a photomicrograph of the poly(amide-tmtde) polymer

wtth the PTFE and graphite addittves. Fine cracks perpendicular to the

sliding direction and many small pits can be seen. The PI-4701 polyimide with

CFX additive produced a slmllar surface with many small pits. Thus,

powdered solld lubricant addltlves tended to produce weak points in the

polymers where cracks could initiate and spans be produced.

It is generally well known that graphite fibers tend to dust tn a vacuum

environment. Dusting was not observed in this study, but the graphite fibers

on the wear track surfaces were observed to crack and debond from the

surfaces. Figure 20(a) gives a photomicrograph of the wear surface of the

polyphenelene sulfide polymer with 40 percent graphite fibers and Fig. 20(b)

gives a similar photomicrograph for the 80 PMOA/20 BTDA poly’tmide with

50 percent graphite ftbers. The polyphenelene sulfide with graphite fibers

shows considerable crumbling and pitting on the wear track; while the

polyimide with graphite fibers shows less crumbllng, but some cracking withtn

the fibers and some shallow surface pitting did occur. In etther case,

graphite fibers did not produce good sl!dtng wear surfaces.

SUMMARY OF RESULTS

Fr’ictlon, wear, and surface morphological stud~es of 10 different polymer

based materials evaluated in a vacuum env~ronment fndicated that:

1. Polymer materials, in general, have a great potential for use as long

term lubricants for applications which occur under vacuum conditions. Very

low wear rates and fr~ction coefficients

vacuum environment.

2. Of the ten polymer materials eva”

polyimlde and the PI-4701 polylmide film

a space environment lubrlcant.

as low as 0.02 were obtained in a

uated, the 80 PMDA/20 BTDA solld body

showed the best potential for use as

14
3. The 100 PMDA polylmide solld or film produced high friction and

relatively low wear In vacuum. Thus, this polymer may have potential for such

applications as space station traction drives.

4. The friction and wear properties of most of the polylmldes in vacuum

were so good that other solid lubricant additives are not necessary.

5. In vacuum, powdered solid lubricant additives to the polymers tended

to produce weak points in the composites where cracks could be initiated and

spallation could occur under sllding conditions.

6. In vacuum, polymers which contained graphite fibers did not produce

good sllding wear surfaces. The f~bers on the surfaces tended to crack and

debond on long sllding durations.

7. All the polymer materials, except the 80 PMDA/20 BTDA polyimlde with

50 percent graphite fibers and the 100 PMDA solid and film, produced lower

frtctflon coefficients In vacuum than in air.

8. Only the polyamides without solld lubricant additives produced

substantially lower steady-state wear rates In vacuum than in air.

REFERENCES

1. Todd, M.J., “Solid Lubrication of Ball Bearings for Spacecraft Mechanism,”

Tribol. Int., 15, 331-337 (1982).

2. Fusaro, R.L., “Molecular Relaxations, Molecular Orientation, and the

Frlctlon Characterlstlcs of Polyimide F~lms,” ASLE Trans., 20, 1-14 (1977).

3. Fusaro, R.L., “Effect of Atmosphere and Temperature on Wear, Friction, and

Transfer of Polyimide Films,” ASLE Trans., 27, 125-133 (1978).

4. Fusaro, R.L., “Low-Wear Partially Fluorinated Polyamides,” ASLE Trans.,

28, 542-552 (1985).

5. Serafini, T.T., Delv~gs, P., and L~ghtsey, G.R., “Thermally Stable

Polylmldes from Soluttons of Monomeric Rectants,” J. Appl. Polym. SC’I.,

16, 905-916 (1972).

15
6. Vaughan, R.W., Jones, R.J., O“Rell, M.K., and Zakrzewskl, G.A.,

“Development of Autoclave Moldable Addition-Type Polyamides,”

TRW-26446-6015-RU-00, NASA CR-134900 (1976).

7. Jones, R.J., O’Rell, M.K., and Horn, J.M., “Polyimldes Prepared from

Perfluorotsopropyl ldene Dlarnlne, ” U.S. Patent 4,111,906 (Sept. 5, 1978).

8. Fusaro, R.L. and Hady, W.F., “Tribological Properties of

Graphite-Fiber-Reinforced, Partially Fluorinated Polylmide Composites,”

ASLE Trans., 29, 214-222 (1986).

9. Fusaro, R.L., “Mechanisms of Lubrication and Wear of a Bonded

Solid-Lubricant Film,” ASLE Trans., 24, 191-204 (1981).

10. Fusaro, R.L., “How to Evaluate Solid Lubricant Films Using a P’in-on-Otsk

Trlbometer,” NASA TM-87236 (1986).

16
WI-ARlRACK~ & ,-RIDER SLANTED

/

. AT 45° TO DISK

t DIRECTION OF

// ~ ROTATION

FIGURE 1. - SCHEMATIC ARRANGEMENT OF FRICTION SPECIMENS.

.50

++$$$+4++

~ .40

z

u

+

z

E~ ,30

w ‘+++++++++++

z

o

; .20 •l 50% RH AIR TEST

v o 0 VACUUM TEST (0.13 PA(10-3TORR))

E o

,10

,L o~ooooo[ooooq Ooooq

o 100 200 300 400

SLIDING DURATION. KILOCYCLES

FIGURE 2. - COMPARISON OF THE FRICTION PROPERTIES OF A POLYPHENLINE SULFIDE SOLID DISK WITH 50 PERCENT ADDED GRAPHITE

FIBERS SLIDING IN 50 PERCENT RH AIR AND IN VACUUM.

17
1

c1

o

A

.50

50% RH AIR TEST

VACUUM TEST (0.13 PA (10-3 TORR))

VACUUMTESTWHICH WAS MOVED TO A NEW TRACK

WITHOUT REMOVING FROM THE VACUUM

SLIDING DURATION 1560 KC

%

o

0

0

~ SLIDING DURATION 380 KC

“~ 00000000

I I

o 20 40 60 80 100 120 140 160

SLIDING DURATION, KILOCYCLES

FIGURE 30 - COMPARISON OF THE FRICTION PROPERTIES OF APOLY

(AMIDE-IMIDE), PTFE AND GRAPHITE COMPOSITE SOLID BODY

SLIDING IN AIR (50 PERCENT RELATIVE HUMIDITY) AND IN VACUUM.

r SLIDING DURATION 860 KC

❑ 50% RELATIVE HUMIDITY AIR TEST

b VACUUM TEST WHICH WAS MOVED TO

A NEW TRACK WITHOUT REMOVING

FROM THE VACUUM

g ,20

L

,10 ~

&ApA SLIDING DURATION 1450 KC

mpmlm -q m~my

o 20 40 60 80 100 120 140 160

SLIDING DURATION, KILOCYCLES

FIGURE 4, - COMPARISON OF THE FRICTION PROPERTIES OF ACOMMERCIALLY AVAILABLE POLYIMIDE SOLID BODYIN VACUUM AND IN

50 PERCENT RELATIVE HUMIDITY AIR.

18
I ❑ 50% RH AIR TEST -

~ ~30~ O VACUUM TEST (0,13 PA (10-5 TORR))

w

z

SLIDING DURATION 1690 KC

E ,20 —

u

o

v [ P

1-

z~

0 100 200 300 400 500 600 700 800 900

SLIDING DURATION, KILOCYCLES

FIGURE 5, - COMPARISON OF THE FRICTION PROPERTIES OF A PMR-15

POLYIMIDE SOLID BODY WITH 70 WEIGHT PERCENT ADDITIVE OF PTFE

IN AIR AND IN VACUUM.

SLIDING DURATION, 540 KC

~ ,60

z

u

++4+++*+4 +6*+++++++

v O VACUUM TEST, 0.13 PA (10-3 TORR)

~ ,40 0 VACUUM TEST, 0.00013 PA (10-6 TORR)

~

v

= 50% RELATIVE HUMIDITY AIR TEST

~ ,20

5

7SLIDING DURATION, 1690 KC

e

/-SLIDING DURATION, 1920 KC

L @%%QaQ)%%@@ %@Q?%%%

o 100 200 300 400

SLIDING DURATION, KILOCYCLES

FIGURE 6, - COMPARISON OF THE FRICTION PROPERTIES OF ACOPOLYIMIDE MADE FROM A 4-BDAF DIAMINE AND 80 PERCENT PMDA

AND 20 PERCENT BTDA DIANHYDRIDES IN AIR AND IN TWODIFFERENT VACUUM ENVIRONMENTS.

19
,20

+

=5

0.

:U

tic .10

~~

IL o

u

o 100 200 300 400

SLIDING DURATION, KILOCYCLES

FIGURE 7, - COMPARISON OF THE FRICTION PROPERTIES OF AN

80 PERCENT 4-BDAF/PMDAAND 20 PERCENT 4-BDAF/BTDA COPOYIMIDE REINFORCED WITH 50 PERCENT BY WEIGHT OF HIGH

MODULUS GRAPHITE FIBERS IN VACUUM AND IN AIR,

i!;:!!$$!v?

)

O VACUIJMTEST, 0.13 PA (10-3 ToRR)

❑ 50% RELATIVE HUMIDITY AIR TEST

F

.,o~

o 20 40 60 80 100 120 140 160

SLIDING DURATION, KILOCYCLES

FIGURE 8. - COMPARISON OF THE FRICTION PROPERTIES OF A 4-

BDAF/PMDA POLYIMIDE SOLID BODY IN VACUUM AND IN 50 PERCENT

RH AIR,

20
.60

-r

O VACUUM TEST, 0.13 PA (10-3 TORR)

+

E

•l AIR TEST, 50% RELATIVE HUMIDITY

v

SLIDING DURATION,KILOCYCLES

FIGURE 9, - COMPARISON OF THE FRICTION PROPERTIES OF A

100 PERC.ENTPMDAPOLYIMIDE FILM IN AIR ANDIN VACUUM,

VACUUM TEST, 0,13 PA (10-3 TORR)

AIR TEST, 50% RELATIVE HUMIDITY

z

o

r P

:.10 000

= SLIDING DURATION,3020 KC L 000000000000000000000

I I I

o 20 40 60 80 100

SLIDING DURATION,KILOCYCLES

FIGURE 10, - COMPARISON OF THE FRICTION PROPERTIES OF A

PI-4701 POLYAMIDE FILM IN AIR AND IN VACUUM.

21
,20

~

w

:[

❑cl %nmm

v

au •1 ❑ ~nnumm ❑abooancl

r

u

u

s ,10

= O VACUUM TEST, 0.13 PA (10-3 TORR)

o

$@+w ❑ AIR TEST, 50% RELATIVE HUMIDITY

r

v

‘~

o 100 200 300 400 500

SLIDIING DURATION, KILOCYCLES

FIGURE 11. - COMPARISON OF THE FRICTION PROPERTIES OF A

PI-4701 POLYAMIDE FILM WITH 50 WEIGHT PERCENT ADDITIVE

OF GRAPHITE FLUORIDE IN AIR AND IN VACUUM,

22
w

3aIw:

z

3(IIWIA10d

V(I181N33ti3dOZ

/V(lWdlN33ti3d08

3(lIWIA10d

VaMdlN33ti3d 001

~+ld lN33ti3d01

3(lIWIA10d51 tiWd

3aIWIA10d

31W11VAV

A11V13U3WW03

s3AIlIaav

1:1

311HdVN9 (INV341d

I f3aIwI 3aIwvj AIOd

SN3814 311HdVti9

lN33ti3doh/3a141ns

%

lN313N3HdA10d
~4

z

o

P

$3

;/, ,0,

0 VACUIJMTEST, 0,13pA (10-3 TORR)

O VACUUM TEST, 0,00013 PA (10-3 TORR)

A

w ■ 50% RELATIVE HUMIDITY AIR TEST

o

52

11 :,~

x

0

(n

~o 500 1000 1500 2000

SLIDING DURATION, KILOCYCLES

FIGURE 13, - DISK WEAR TRACK CROSS SECTIONAL AREA AS AFUNCTION OF SLIDING DISTANCE FOR A 440 C HT STAINLESS STEEL

PIN SLIDING AGAINST ACOPOLYIMIDEMADE FROM A 4-BDAFDIAMINE AND 80 PERCENT PMDAAND 20 PERCENT BTDADIANHYDRIDES,

,-9—

O VACUUM TEST, 0.13 MPA (10-3 TORR) I

■ 50%RH AIR TEST

—

—

—

50 100 150 200 250 300

SLIDING DURATION, KILOCYCLES

FIGURE 14, - DISK WEAR TRACK cRoSs SECTIONAL AREA AS AFUNCTION OF SLIDING DURATION FOR A 440 C HT STAINLESS STEEL

PIN SLIDING ON A 100 pERCENT 4-BDAF/PMDA SOLID DISK.

24
‘= IOOXIQ-9 n

.

80

60

40

20

1

VACUUM TEST, 0.13 PA (10-3 TORR)

50% RH AIR TEST

VACUUM TEST MOVED TO A NEW TRACK

ON DISK WITHOUT REMOVING FROM

VACUUM

o 200 400 600 800 ‘1000 1200 1400 1600

SLIDING DURATION. KILOCYCLES

FIGURE 15, - DISK WEAR TRACK CROSS SECTIONAL AREA AS AFUNCTIONOF SLIDING DURATION FORA440 C HTPINSLIDING ON A

COMMERCIALLY AVAILA8LE POLYIMIDE SOLID DISK,

25
3AIlIaav

’43 lN33ti3d 05

3aIWIA10d 10Lh-Id

3aIWIA10d 10Zh-Id

I

3(lIWIA10d

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3aIWIA10d

ValH lN33ti3dOZ

/V(IWdlN33ti3d08

I

3aIWIA10d

VaWd lN33N3d 001

I

I

n

o

m

3Ald lN33ti3dOL

3aIWIA10d Sl tiWd

I

3aIWIA10d 31W

-11VAVA11V13N3WW03

\\\\\\\\\\\\,

\ S3AIlIaW

311HdVN9 aNV ~jld

(3aIWI-3(.lIWVIAIOd

w

>

SN3H14

311HdVN9 lN32ti3dOh

I 3a131flS3N313N3HdA10d —

N m 3 m m

I I

o

I

‘o o

I

o 0

9NIaIls *w/~w ‘31VN NV3M

26
440 C HT STAINLESS STEEL SLIDING

ON ITSELF IN AIR

.50 — 2

●

■

~ ,40 — ~ HIGHFRICTIONS LOW WEAR POLYAMIDES

z

L .30 —

~

~ .20 —

P

6

g ,10 —lo 9

&e LOW FRICTION

LL 7;; LOW WEAR POLYAMIDES

11* ●1*5

~o-16 ,.-15 ,.-14 ,.-13 ,.-12 ,.-ll ,0-10 ,0-9 ,.-8

WEAR RATE, M3/MOF SLIDING

FIGURE 17, - FRICTION COEFFICIENT AS A FUNCTION OF POLYMER

WEAR RATE FOR VARIOUS POLYMER MATERIALS SLIDING IN A

VACUUM OF 0.13 pA, (11 100 PERCENT PMDAPOLYIMIDE FILM,

(2) 100 PERCENT PMDAPOLYIMIDE SOLID, (3) PI-4701 POLYIMIDE/50 PERCENT CFX FILM, (4) POLY (AMIDE-IMIDE)/PTFE

AND GRAPHITE SOLID, (5) COMMERCIALLY AVAILABLE POLYIMIDE

SOLID, (6) GRAPHITE FIBER REINFORCED POLYAMIDE SOLID,

(7) POLYPHENELENE SULFIDE/50 PERCENT GRAPHITE FIBER SOLID,

(8) PMR-15 POLYIMIDE/70 PERCENT PFTE SOLID, (9) PI-4701

POLYIMIDE FILM, (10) 80 PERCENTPMDA/20 PERCENT POLYAMIDE

SOLID, (11) 80 PERCENT PMDA/20 PERCENT BTDA POLYIMIDE

SOLID IN AO.00013 PA VACUUM,

27
(A) POLYIMIDE (80% PMDA\20% BTDA).

(B) [’OLYIMIDE(100% PMDA).

FIGURE 18. - HIGH MAGNll”Ii;ATIONPHOTOMICROGRAPHS OF THE WEAR

TRACKS OBTAINED IN VACUUM ON TWO DIFFERENT POLYIMIDE FILMS.

28
(Al PMR-15 POLYIMIDE POLYMER WITH PTFE ADDITIVE,

(B) POLY (AMIDE-IMIDE) POLYMER WITH PTFE AND GRAPHITE

ADDITIVES,

FIGURE 19, - HIGH MAGNIFICATION PHOTOMICROGRAPHS OF THE WEAR

TRACKS OBTAINED IN VACUUM ON POLYMER DISKS WITH POWDERED

SOLID LUBRICANT ADDITIVES.

29
(A) POLYPHENELENE SULFIDE.

(B) POLYIMIDE (80% PMDA/20% BTDA).

FIGURE 20. - HIGH MAGNIFICATION PHOTOMICROGRAPHS OF THE WEAR

TRACKS OBTAINED IN VACUUM ON GRAPHITE FIBER REINFORCED

POLYMER DISKS.

30
1. Report No. 2. Government Accession No.

NASA “IM--88966

4. Title and Subtitle

Tribologlcal, Properties of Polymer Films and Solid

Bodies in a Vacuum Environment

7. Author(s)

Robert l.. Fusaro

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

12, Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

I

15. Supplementary Notes

3. Recipient’s Catalog No

5. Report Date

6, Performing Organization Code

505-63-01

8. Performing Organization Report No.

E-3429

O. Work Unit No

1. Contract or Grant No.

3. Type of Report and Period Covered

Technical Memorandum

4. Sponsoring Agency Code

Prepared for the 1987 Annual Meeting of the American Society of Lubrication

Engineers, Anaheim, California, May 11-14, 1987.

6. Abstract

lhe tribological properties of ten different polymer based materials were evaluated in a vacuum environment to determine their suitability for possible lubrication applications In a space environment, such as might be encountered on the

proposed space stat~on. A pin-on-disk tribometer was used and the polymer materials were evaluated either as solid body disks or as films applied to 440C HT

stainless steel disks. A 440C HT stainless steel hemispherically tipped pin was

slid against the polymer materials. For comparison, similar tests were conducted

in a controlled air atmosphere of 50 percent relative humidity air. In most

instances, the polymer materials lubricated much better under vacuum conditions

than in air. lhus, several of the materials show promise as lubricants for

vacuum applications. Friction coefficients of 0.05 or less and polymer material

wear rates of up to 2 orders of magnitude less than found in air were obtained.

One material showed considerable promise as a traction drive material. Relatively high friction coefficients (0.36 to 0.52) and reasonably low wear rates

were obtained in vacuum.

___

7 Key Words (Suggested tjy A~!hor(s)) 16. Distribution Statement

Solid lubricant; Polymers; Fricti[jn; Uncla~sif ied - unlimited

Wear, Vacuum, Space lubrication; S[AR C,~te[jory2“/

Composites

I

3. Secur!ty Class lf. (of this l~;port) ~. Security Classif (of this page)

Unclassified

– -------;s~~~-~iTo3 i

1

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